Coatings have been applied to a variety of surfaces for many purposes including corrosion resistance. In the electroplating, coating, and painting industry, a coating's effectiveness as a barrier to a corrosive environment is determined by measuring the coating's thickness, porosity, and/or corrosion rate. For instance, different thickness specifications exist for different types of coatings. ASTM Standard B733, for example, gives specifications for electroless nickel coatings, including minimum coating thickness for mild, moderate, severe, and very severe service. On the other hand, coating porosity and corrosion rate specifications for a particular coating are determined by the level of protection needed to minimize component failure or to reduce contamination in a particular industrial application or corrosive environment. Thus, measurement of these parameters provides useful information on the quality of the coating and can ensure long coating life without premature coating failure.
It is common practice to determine the quality of a particular coating by visual inspection. For example, in the case of a coating on the inside of a gas cylinder, this is performed either with the naked eye or by using a boroscope that is connected to a computer for further image processing. Visual inspection is an adequate technique to identify grossly damaged or grossly out-of-specification coatings. However, coating defects such as high porosity, micro-cracks, or insufficient thickness cannot be determined visually. Such defects or noncompliance to coating specifications are due to variations or possible problems in the coating process, or due to wear. And since they cannot be determined visually, other techniques must be used to monitor the quality of the coating and ensure that a consistent, in-specification coating is present.
The thickness of a coating can be determined by several methods known in the art. Some common methods of measuring a coating thickness include the coulometric, beta-backscatter, eddy-current, magnetic, and optical measurement method. These methods are usually classified as either destructive or non-destructive.
Destructive methods require destroying the coating or the coated object in order to measure coating thickness. On the other hand, non-destructive methods involve measuring a coating thickness without destroying the coating or the coated object, i.e., leaving the coating or the coated object intact, and usable or reusable.
The coulometric and optical methods are considered destructive techniques. The coulometric method involves electrolytically dissolving the coating to determine the coating thickness. The optical method involves sectioning the coated object and examining the coating cross-section under a microscope.
In contrast, the beta-backscatter, eddy-current, and magnetic methods are considered non-destructive because they measure a coating thickness by determining the change in some physical characteristic that does not require destroying the coating or the coated object, such as measuring the change in the magnetic field due to the presence of the coating.
Likewise, coating porosity and corrosion rate can be measured by destructive or non-destructive techniques. Destructive techniques include chemical porosity tests, accelerated salt-spray tests, and long-term exposure tests. Non-destructive techniques include potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).
Currently, the most commonly used technique for measuring the thickness, porosity, and corrosion rate of a coating inside a gas cylinder involves destructively sectioning a portion of the cylinder to provide access to its interior surface so that the measurements can be made. This is because commercially available, nondestructive measuring devices are not designed to make measurements in the confined geometry of a gas cylinder which only has a small opening. In addition to rendering the gas cylinder unusable, sectioning takes a relatively long time and would be unsuitable for production-run quality control of a coating process.
Generally, the non-destructive methods have been designed as bench-lab techniques and usually accommodate small, exterior-coated samples. The ultrasonic and magnetic methods, however, have been employed in hand-held instruments with small probes that can be used in the field to measure the thickness of a coating on large structures such as bridges, piping, and tanks. Magnetic testers have also been commercially developed with telescopic arms to measure the coating thickness in hard-to-reach places, such as under highway bridges or inside process piping.
The telescopic arm of these magnetic testers, however, cannot effectively reach inside a hollow body with a single small opening, such as a gas cylinder or a pressure vessel, to make a measurement. Thus, objects having such a geometry present a unique technical problem. The problem arises because the coating test probe employed by these techniques not only has to fit inside the hollow body through a narrow opening, but must also make physical contact with the coated surface inside the hollow body within certain angles. Commercially available telescopic arm probes may fit into the hollow body. However, they are not flexible enough to make physical contact with the interior surface of the hollow body having a small diameter opening at a perpendicular angle to make a coating measurement.
Similarly, portable electrochemical equipment has been designed to allow corrosion rate measurement in process piping. However, the equipment requires special probes that are screwed into the side of the pipe to allow access to the interior, corrosive liquid. The measured corrosion rate of the probe is then correlated to the corrosion rate of the interior piping material or coating. Obviously, attaching such a probe to a gas cylinder would also be destructive to the gas cylinder.
In view of the foregoing, there is a need in the art for a non-destructive method and apparatus for measuring the quality of a coating inside a hollow body having a narrow or a small opening. There is also a need for a method and apparatus for measuring the quality of the coating in such hollow bodies accurately and in a short period of time.
Accordingly, it is an object of the present invention to provide a non-destructive method and apparatus for measuring the quality of a coating inside a hollow body having a narrow or a small opening.
It is a further object of the present invention to provide a method and apparatus for measuring the quality of the coating in such hollow bodies accurately and in a short period of time.
These and other objects of the present invention will become apparent in light of the following specification, and the appended drawings and claims.